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The Structure and Physicochemical 553 The Canadian Mineralo gist Vol.32, pp. 553-561 (1994) THESTRUCTURE AND PHYSICOCHEMICALCHARACTERISTICS OFA SYNTHETICPHASE COMPOSITIONALLY INTERMEDIATE BETWEENLIEBIGITE AND ANDERSONITE RENAI.ID VOCHTEN, LAURENT VAN HAVERBEKE ANDKAREL VAN SPRINGEL Iztboratortumvoor chemischeen fysische mineralagie, IJniversiteit Antwerpen (RUCA), MiddelheimlaanI, 8-2020 Antwerpm, Belgium NORBERT BLATON AND OSWALD M. PEETERS Laboratoriumvoor analytischechemie en medicbnlefysicochemie, Instituut voor FarmaceutischeWetenschnppen Kathnlieke Universiteitlzuven, VanEvenstraat 4, 8-3000 Leuven,Belgium ABSIRACT During the s1'nthesisof liebigite and andersonite,starting from tetrasodiumuranylcarbonate and calcium ions, an unknown stableand crystalline intermediatephase is formed at a Ca2*/Na*ratio lying betweenthose necessary to form the above-cited compounds.Chemical analysis of this yellow-green intermediatephase results in the formula: Ca1.rNa6.6r[UO2(COr)3].rH2O ("r = 5.38). The scanningelectron micrographs show foliated lathlike crystals.From single-crystatAata tni cryitats arb found to be orthorhombic,a 18.150(3),b 16.866(6),c 18.436(3)A, spacegroup Pnnm. The basicstrucrural units consisrof three UO2(CO3)3units. Each U atom is [8]-coordinated,resulting in a distorted hexagonal-bipy'ramidalpolyhedron. A chemical analysisindicates that the structureis short of positive charges,which could be balancedby the presenceof oxonium ions. Resultsof the crystal-structureanalysis support this proposal,which ensuresa tight fit of the water moleculethat is not attachedto either Nar or Ca2+ions. The powder-diffractionspectrum calculated from the single-crystalstructffe corresponds very well with tle measuredone. The fluorescenceand phosphorescencespectra are given, as well as information on the thermaldecomposition, as revealed by thermogravimetricand differential scanningcalorimetric analyses. Keywords:liebigite, andersonite,synthetic phase, crystal structure,physicochemical properties. SoMraans Au cours de nos synthbsesde la liebigite et de l'andersonite,i partir de I'uranylcarbonatede tetrasodiumet d'ions de calcium commemat6riaux de d6part,nous avonsd6couvert une phasemdconnue, stable et cristalline,ayant un rapporr ga2+/Na*entre ceux qui s'avdrentn6cessaires pour former les deux autrescompos6s. Une analysechimiqui de cene phase interm6diairejaune-vert mdne i la formuleCar.*Nan.ur[UO2(CO3)3].xH2O (.r= 5.38). D'aprbs les photospriies au microscope 6lectroniqueb balayage,il s'agit de cristauxen plaquettesayart une sructure en feuillets. Les donnfesobienues par diffraction X sur cristalunique montrent que cettephase est orthorhombique,a 18.150(3),b 16.866(6),c 18.436(3)A, groupespatial Pnnm. Les 616mentsstructuraux fondamentaux comprennent rois unit6s I stoechiom6trieUor(COr).. Chaqueatome d'uranium possbdeune coordinencehuit et loge dans un polybdre hexagonalbipyrarnidal difformi. Uni anatysechimique montre que la structureaccuse un d6ficit de chargespositives, qui pounait 0tre satisfait par la prdsenced'ions oxonium. Les r6sultatsde l'6bauche de la structurecristalline confiflnent cette hypothdse,qui assureun adaptationserr6e de la mol6cule d'eau non rattachdeau Nat ou au Ca2*.Le spectrede diffraction X (m6thodedes poudres) calcul6 i partir desrdsultats obtenus sur cristal unique concordetrbs bien avec le spectremesur6. Nous d&rivons aussiles spectresde fluorescenceet de phospho- rescence,de mdmeque les r6sultatsde la d6compositionthermique par thermogravim6trieet par calorim6triediffdrentielle en modebalayage. (Traduit par la R6daction) Mots-cl6s:liebigite, andersonite,phase synth6tique, structurc cristalline, propri6t6s physicochimiques. 554 THE CANADIAN MINERALOGIST INTRoDUcrtoN According to the stability diagramsof Alwan & Williams (1980),the two uranyl carbonateminerals, liebigite,Ca2[UO2(CO3)2]'10H2O, and andersonite, CaNa2[UO2(CO3)2]'6H2O,should be expectedto co- exist, Severalsyntheses of theseminerals have been described(Blinkhoff 1906,Axelrod et al. 1951, Bacheletet aL \952, Meyrowitz& Ross 1961, Meyrowitzet al.1963, dejka 1969,Coda et at. 1981). Meyrowitz et al. (L963) mentionedthe formation of unknown yellow-green crystalsobtained during the synthesisof liebigite from potassiumcarbonate, uranyl nitrate and calcium nitrate. In a previouspaper, Vochten et al. (1993) described a new methodfor the synthesisof liebigite and ander- sonite starting from tetrasodium uranyl carbonate Naa[UO2(CO:)z](NaUTC). As mentionedin that paper,the formation of an intermediatephase, consist- ing of well-formed yellow-green lath-like crystals, was observed.The X-ray powder-diffractionpattem of this phasecould not be matchedwith that of any known compound. For the presentsynthesis, two grams of NaUTC were dissolvedin 100 mL of 0.04 M CaCl2and left to evaporatein an openvessel at 25oC.After one week, well-formed hemisphericalcrystals of liebigite started to grow. After two weeks,the formation of crystalsof the intermediatephase was observed,and after three weeks, crystals of andersonitewere formed. The aim of the presentstudy was to determinethe chemical compositionand the crystal structureof the unknown syntheticphase, as well as someof its physicochemi- cal characteristics. CRYSTAL MoRPHoLocY The scanningelectron micrographs (Fig. 1) show that the crystals are orthorhombic,with a lath-like habit and facetededges. The length of the crystals rangesfrom 0.6 to 1.0mm, andtheir width, from 0.01 to 0.1 mm. As cleady shownin the SEM photographs, the crystalsare built up of subparallellayers. Frc. l. Scanningelectron micrographs of theintermediate The morphology of the crystals can be considered phasebetween liebigite and andersonite. Scale bar: A' as a combinationof the forms {010} and {001}. The B: 100pm; C: 10Pm. two othervisible forms, {01I } and {0Tl }, are barely developed.The layering in the photographsis parallel to (001). TGA 951 apparatuswas used, with an applied rate of heatingof S'C/min and a constantflow of N2 of CmurcatCowosmor 30 ml/min. In orderto detectthe temperatureat which CO2 is liberated, the outlet of the gas streamwas The Na, Ca and U content was determinedby passedthrough a Ba(OH)2solution. The resultsare electron-microprobeanalysis. Oligoclase (Na), wollas- shown in Figure 2. The thermal decompositionof the tonite (Ca) and UO2(U) were usedas standards.Data carbonategroups starts at approximately300oC and is were collected at 12 different points. The water and completeby 800'C, with a loss of 2lVo by weight, carbon dioxide contentwas determinedby thermo- correspondingto three carbonategroups. It is clear gravimetry (TG) in combination with differential from the TG curve that the dehydrationand decar- scanningcalorimetry @SC). A DupontDSC 910 and bonation stepsoverlap, so that it is quite difficult to SYNT}TETICPIIASE BETWEENLIEBIGITE AND ANDERSONTIE 555 Omcar,PanarcrBns The crystals are biaxial negative,with 2V = 65' (t5'), and have a positive elongationand parallel extinction. The indices of refraction are s.1.525 (t0.005), P 1.s47(10.002) and 1 1.556(10.002). The indices p and y were determinedby meansof liquids with known index of refraction. The value of cr was calculatedfrom B, y and 2V. Most of the crystals are clearly twinned, as can be detectedby polarizing microscopy. LulanrcscrNcnSpecrRA Both fluorescenceand phosphorescencespectra were recordedby meansof a Perkin-Elmer MPS44B specuofluorimeterat298 and77 K, with an excitation wavelengthof 380 nm, a bandpassfor excitationof 20 nm and for emissionof 5 nm, and a scanspeed of 120nm/min. 2ao 4oo 8oo looo r*p1*mfq Frc. 2. Resultsof thermogravimetric(TG) anddifferential scanningcalorimetry @SC) analyses of theintermediate phasebetween liebigite and andersonite. z a determine C the temperatureintervals over which the o individual molecules of water are lost. Only the total o amountof watercould be estimated(l5.4%o); it corre- c spondsto 5.38 water molecules.Table I summarizes -a the compositionin terms of oxides,from which the chemicalformula is calculatedby the classic"residual oxygen"method, based on I I oxygenatoms. This leadsto the formula: Ca1.raNan.63[UO2(COr)3]..xH2O (-t= 5.38). Becausethe electroneutralityrule is to be obeyed, the formula lacks 0.29 positive charges.This problem can be solved by the presenceof a proton, associated with an oxygenatom. This could be an oxygenof UO, or an oxygenofH2O, as hasbeen suggested for other uranium minerals by Smith et al. (1957), Brasseur (1962) and Sobry & Rinne (1973). In this case,the formula can tlen be written as: Ca1.r4Nq.6:(H:O)o.zs [uo2(co3)3]-5H2o. TABLE 1. CI{EMICAL OOTIPOSITIOI\ OF TI{E INTERMEDIATE COMPOUND oxide weigbr% quantities ratio Na2O 3'.12 0.0503 0.63 Emlsslon(nm) u03 47i22 0.1651 1.04 CAZ 2L00 0.472r 3.00 Frc. 3. Luminescencespectra of the intermediatephase HzO t5.36 0.8533 5.38 betweenliebigite and andersonite. Total 100.46 A. Fluorescencespectra at77 and298K. B. PhosphorescencespectraatTT K. 556 T}IE CANADIAN MINERAIOGIST TAII.B 2 REI,IITYE INTENSIIIESOF TgP PHOSPHORESCEN(EBANDS AT 7r K width 1.05' in 35 stepsplus cr1-cr,dispersion, rcR LIEBIoTTETllE S!'T{THS[CINTERMEDIAIE @MFOTJND, AND ANDERSONITE measuringtime/step between 0.5 and 2 s) was usedto collect 12 632 reflectionswith indices-2 < h <26, -2 < k < 24,-2 < I 3 26 and20^u = 60o. Of thesemeasured reflections, 12 602 wereaccepted, htermedistecompound 8.0 wereconsidered independent reflections. Of Andemnite 3.5 and9 014 the 9 014 uniquereflections (R6, = 0.018),4 861 with I )2o. [o(I) from counting statistics]were considered obserVed(Stoe 1988a).The standarddeviation
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